1. Field of the Invention
The present invention relates to an LPP (laser produced plasma) type EUV (extreme ultraviolet) light source apparatus generating extreme ultraviolet light which is used for exposing a semiconductor wafer or the like.
2. Description of a Related Art
Recently, along with a finer semiconductor process, optical lithography has been making a rapid progress for realizing a finer pattern, and is now required to realize a fine process at 60 nm through 45 nm and further a fine process at 32 nm and beyond in the next generation. Accordingly, it is expected to develop, for example, an exposure equipment using a combination of an EUV light source generating extreme ultraviolet (EUV) light with a wavelength of approximately 13 nm and a reduced projection reflective system in order to cope with the fine process at 32 nm and beyond.
There are three types of EUV light sources including an LPP (laser produced plasma) light source using plasma which is generated by application of a laser beam onto a target, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using synchrotron orbital radiation light. Among these light sources, the LPP light source is considered to be a good candidate for the EUV lithography light source which is required to have a power of a hundred or more watts. This is because of advantages thereof such as one that the LPP light source can provide extremely high luminance close to that of black body radiation since plasma density can be made considerably high therein. The LPP light source also can emit only light within a desired waveband by selecting a target material, and forms a point light source which has an almost isotropic angular intensity distribution and provides an extremely great collection solid angle like 2π to 4π steradians, since there is no structure surrounding the light source such as electrodes.
FIG. 37 is a diagram showing an outline of a conventional LPP type EUV light source apparatus. As shown in FIG. 37, this EUV light source apparatus is configured with a driver laser 101, an EUV light generation chamber 102, a target material supply unit 103, and a laser beam focusing optics 104, as main constituents.
The driver laser 101 is an oscillation-amplification (Master Oscillator Power Amplifier) type laser apparatus generating drive laser beam used for exciting a target material.
The EUV light generation chamber 102 is a chamber in which the EUV light is generated, and is made vacuum therein by a vacuum pump 105 for turning the target material easily into plasma and preventing the EUV light from being absorbed. In addition, the EUV light generation chamber 102 is provided with a window 106 attached thereto for transmitting a laser beam 120 generated in the driver laser 101 to the inside of the EUV light generation chamber 102. Further, a target injection nozzle 103a, a target collection cylinder 107, and an EUV light collector mirror 108 are disposed within the EUV light generation chamber 102.
The target material supply unit 103 supplies a target material used for generating the EUV light to the inside of the EUV light generation chamber 102 via the target material injection nozzle 103a which is a part of the target material supply unit 103. The target collection cylinder 107A collects a remaining part of the supplied target material, which becomes unnecessary without being irradiated with the laser beam.
The laser light focusing optics 104 includes a mirror 104a reflecting the laser beam 120 emitted from the driver laser 101 in the direction of the EUV light generation chamber 102, a mirror adjustment mechanism 104b adjusting the position and angle (tilt angle) of the mirror 104a, a collector element 104c focusing the laser beam 120 reflected by the mirror 104a, and a collector element adjustment mechanism 104d moving the collector element 104c along the optical axis of the laser beam. The laser beam 120 focused by the laser beam focusing optics 104 is transmitted through the window 106 and a hole formed in the center part of the EUV light collector mirror 108 and reaches a path of the target material. In this manner, the laser beam focusing optics 104 focuses the laser beam 120 so as to form a focus on the path of the target material. Thereby, the target material 109 is excited into plasma and an EUV light 121 is generated.
The EUV light collector mirror 108 is a concave mirror which has a Mo/Si film formed on the surface thereof for reflecting light with a wavelength of 13.5 nm, for example, in a high reflectance, and focuses the generated EUV light 121 to an IF (intermediate focusing point) by the reflection. The EUV light 121 reflected by the EUV light collector mirror 108 is transmitted through a gate valve 110 provided to the EUV light generation chamber 102 and a filter 111 which eliminates unnecessary light (electro-magnetic wave (light) with a wavelength shorter than the EUV light and light with a wavelength longer than the EUV light (e.g., ultraviolet light, visible light, infrared light, etc.)) from the light generated from the plasma and transmits only the desired EUV light (e.g., light with a wavelength of 13.5 nm). After that, the EUV light 121 focused on the IF point (intermediate focusing point) is guided to an exposure unit or the like via a transmission optics.
Large energy is radiated from the plasma generated within the EUV light generation chamber 102, and this radiation increases the temperature of the components within the EUV light generation chamber 102. There is known a technique preventing such a temperature rise of the components.
For example, Japanese Patent Application Laid-Open Publication No. 2003-229298A discloses an X-ray generation apparatus including an X-ray source which turns a target material into plasma and radiates an X-ray from the plasma, and a vacuum chamber which accommodates the X-ray source, wherein an inner wall formed with a material having a high absorption rate for an electro-magnetic wave in the range from infrared light to an X-ray is provided within the vacuum chamber. In this X-ray generation apparatus, it is possible to prevent the components within the vacuum chamber from being unnecessarily heated by the radiation energy which is reflected and scattered by the inner wall of the vacuum chamber.
Meanwhile, the plasma generated within the EUV light generation chamber 102 shown in FIG. 37 is diffused as time elapses and a portion thereof flies apart as atoms and ions. These atoms and ions are called debris and radiated to the inner wall and a structure within the EUV light generation chamber 102.
The following phenomena can be caused by the above radiation of the debris flying from the plasma and the electro-magnetic wave radiated from the plasma.
(a) The atoms flying from the plasma adhere to the surface of the window 106 on the inner side of the EUV light generation chamber 102. The laser beam 120 is absorbed by the atoms adhered to the surface of the window 106 on the inner side of the EUV light generation chamber 102 in this manner.
(b) The ions flying from the plasma are radiated to the surface of the window 106 on the inner side of the EUV light generation chamber 102 and the surface of the window 106 on the inner side of the EUV light generation chamber 102 is deteriorated (the surface is made rough and becomes unsmooth). Thereby, the window 106 becomes to absorb the laser beam 120 emitted from the driver laser 101.
(c) The ions flying from the plasma are radiated to the inner wall and the structure of the EUV light generation chamber 102. By the sputtering, the atoms flying from the inner wall and the structure of the EUV light generation chamber 102 adhere to the window 106 on the inner side of the EUV light generation chamber 102. The laser beam 120 is absorbed by the atoms adhered to the window 106 on the inner side of the EUV light generation chamber 102 in this manner.
(d) The material of the window 106 is deteriorated by the absorption of an electro-magnetic wave (light) generated from the plasma and having a short wavelength. Thereby, the window 106 becomes to absorb the laser beam 120.
(e) When the operation period of the EUV light source apparatus becomes long to some extent, the material of the window 106 is deteriorated or damaged by application of the laser beam 120 during the operation period. Thereby, the window 106 becomes to absorb the laser beam 120.
Occurrences of the phenomena of above (a) to (e) cause reduction in energy for turning the target material into plasma and reduction in generation efficiency of the EUV light 121.
In addition, when the window 106 and the atoms adhered to the window 106 absorb the laser beam 120, the temperature of the window 106 increases and the substrate (base material) of the window 106 is distorted, resulting in reduction of the beam focusing capability. Such a reduction of the beam focusing capability invites a further reduction in the generation efficiency of the EUV light 121. Further, the large distortion in the substrate of the window 106 finally invites the breakage of the window 106.
Note that a part of the laser beam focusing optics 104 (e.g., lens, mirror, etc.) is sometimes disposed within the EUV light generation chamber 102. In such a case, the above phenomena of (a) to (e) can be caused also in the part of the laser beam focusing optics 104 disposed within the EUV light generation chamber 102. In particular, in the case that the mirror reflecting the laser beam is disposed within the EUV light generation chamber 102, the above phenomena of (a) to (e) caused in the mirror reduces a laser beam reflectance of a reflection enhancement coating on the reflection surface of the mirror. Thereby, the energy for turning the target material into plasma is reduced and the generation efficiency of the EUV light 121 is reduced.
When the above phenomena of (a) to (e) occur and the window 106 or the laser beam focusing optics 104 is deteriorated, it is necessary to replace the deteriorated optical element with a new optical element.
However, since the laser beam 120 is focused onto the plasma generation position (onto the path of the target material) within the EUV light generation chamber 102, there arises a problem that it is difficult to know whether the window 106 or the laser beam focusing optics 104 is deteriorated or not and to take a rapid response action (replacement of the optical element).
Meanwhile, in addition to the deterioration of the window 106 or the laser beam focusing optics 104, a focusing position (focus) shift of the laser beam 120 is pointed out as a factor inviting instability of the plasma generation and finally changing or reducing the generation efficiency of the EUV light 121. The focusing position shift of the laser beam 120 is caused by an alignment shift of the laser beam focusing optics 104, a pointing shift of the driver laser 101, or the like. The alignment shift of the laser beam focusing optics 104 is mainly caused when an optical element included in the laser beam focusing optics 104 or an optical element holder holding such an optical element bears a thermal burden and the optical element or the optical element holder is deformed, along with the operation of the EUV light source apparatus. Further, the pointing shift of the driver laser 101 is mainly caused when an element or a component within the driver laser 101 bears a thermal burden and the element or the composition member is deformed along with the operation of the EUV light source apparatus.
When the focusing position shift of the laser beam 120 is caused as described above, a focusing spot size or an intensity distribution becomes inappropriate at the plasma generation position (on the path of the target material), or the laser beam 120 is deflected from the target material. Thereby, instability of the plasma generation is invited finally resulting in variation or reduction in the generation efficiency of the EUV light 121.
Note that the focusing position shift of the laser beam 120 can be repaired by readjustment of the alignment in the laser beam focusing optics 104, without replacing the optical element. Thereby, the focusing position of the laser beam 120 can be returned to the original position (plasma generation position) and it is possible to stabilize the plasma generation and resultantly to recover the generation efficiency of the EUV light 121 to the original value.
However, since the laser beam 120 is focused to the inside of the EUV light generation chamber 102 (plasma generation position), there is a problem that it is difficult to know whether the focusing position of the laser beam 120 is shifted or not, and to take a rapid response action (readjustment of the alignment in the laser beam focusing optics 104).